To prepare vinaigrette, whisk together all ingredients. Add salt and black pepper to taste.
To prepare salad, combine 4 cups water, lentils, and bay leaves in a medium saucepan; bring to a boil. Reduce heat; simmer 18 to 20 minutes. Drain; discard bay leaves. Gently stir together lentils, celery, onion, parsley, and salt. Add tomatoes and 2 tablespoons chives. Drizzle with vinaigrette; toss gently to coat. Divide watercress evenly among 6 plates; top with 3/4 cup lentil mixture. Top with remaining 2 tablespoons chives.